Rigid polyurethane and polyisocyanurate foams are produced using cell-stabilizing additives to ensure a fine-celled, uniform and low-defect foam structure and hence to exert an essentially positive influence on the performance characteristics, particularly the thermal insulation performance, of the rigid foam. Surfactants based on polyether-modified siloxanes are particularly effective and therefore represent a preferred type of foam stabilizer.
Since there are a multiplicity of different rigid foam formulations for different fields of use where the foam stabilizer has to meet individual requirements, polyether siloxanes of varying structure are used. One of the selection criteria for the foam stabilizer is the blowing agent present in the rigid foam formulation.
There have already been various publications concerning polyether siloxane foam stabilizers for rigid foam applications. EP 0 570 174 B1 describes a polyether siloxane of the structure (CH3)3SiO[SiO(CH3)2]x [SiO(CH3)R]ySi(CH3)3, the R radicals of which consist of a polyethylene oxide linked to the siloxane through an SiC bond and end-capped at the other end of the chain by a C1-C6 acyl group. This foam stabilizer is suitable for producing rigid polyurethane foams using organic blowing agents, particularly chlorofluorocarbons such as CFC-11.
The next generation of chlorofluorocarbon blowing agents are hydrochlorofluorocarbons such as, for example, HCFC-123. When these blowing agents are used for rigid polyurethane foam production, it is polyether siloxanes of the structural type (CH3)3SiO[SiO(CH3)2]x[SiO(CH3)R]ySi(CH3)3 which are suitable according to EP 0 533 202 A1. The R radicals in this case consist of SiC-bonded polyalkylene oxides which are assembled from propylene oxide and ethylene oxide and can have a hydroxyl, methoxy or acyloxy function at the end of the chain. The minimum proportion of ethylene oxide in the polyether is 25 percent by mass.
EP 0 877 045 B1 describes analogous structures for this production process which differ from the first-named foam stabilizers in that they have a comparatively higher molecular weight and have a combination of two polyether substituents on the siloxane chain.
The production of rigid polyurethane foams using purely hydrofluorocarbons, e.g., Freon, as a blowing agent may, according to EP 0 293 125 B1, also utilize mixtures of different stabilizers, for example, the combination of a purely organic (silicon-free) surfactant with a polyether siloxane.
A more recent development in the production of rigid polyurethane foams is to dispense with halogenated hydrocarbons as blowing agents entirely and to use hydrocarbons such as pentane instead. EP 1 544 235 describes the production of rigid polyurethane foams using hydrocarbon blowing agents and polyether siloxanes of the already known structure (CH3)3SiO[SiO(CH3)2]x[SiO(CH3)R]ySi(CH3)3 having a minimum chain length for the siloxane of 60 monomer units and different polyether substituents R, the mixed molecular weight of which is in the range from 450 to 1000 g/mol and the ethylene oxide fraction of which is in the range from 70 to 100 mol %.
DE 10 2006 030 531 describes the use of polyether siloxanes, as foam stabilizers in which the end group of the polyethers is either a free OH group or an alkyl ether group (preferably methyl) or an ester. Particular preference is given to using such polyether siloxanes which have free OH functions. The use of the specific polyether siloxanes is said to exert a positive influence on the fire behaviour.
U.S. Pat. No. 4,014,825 describes organomodified siloxanes for polyurethane foam production which, in addition to alkyl and polyether substituents, also bear side chains having tertiary OH groups. Thus, additional substituents are introduced in the '825 patent. The polyethers used in the '825 patent are usually methyl endblocked. Generally, the polyethers do not have a specific arrangement of the alkylene oxide units, and so there is no defined OH functionality in the case of a non-endblocking.
U.S. Pat. No. 4,746,683 describes improving the open-cell content of high resiliency flexible foams by using polyether siloxanes wherein a high proportion of the polyethers bear secondary or tertiary OH groups. The siloxanes contain not more than 10 silicon atoms and the polyethers consist of 3 to 13 oxyalkylene units.
Yet the foam stabilizers described in the aforementioned publications do not offer the desired optimal foam properties over the whole spectrum of the various rigid foam formulations, and there are many fields where improvements in foam stabilizers over the prior art are desirable in order to further optimize the performance characteristics of rigid foams, particularly in respect of thermal conductivity and foam defects at the surface.
It is typically the foam defects at the surface which are increasingly becoming the center of attention. In the case of refrigerators and metal composite elements (wall elements for the construction of buildings) for example, where polyurethane foam is faced with layers of steel sheet, voids in the foam which are directly underneath the face layer can be visible as bulges or blisters on the surface of the face layer and thus convey an impression of poor quality to an observer. In addition to the visual impression, physical characteristics also suffer when such foam defects are present. For example, face layer adherence and thermal insulation performance generally worsen in their initial values and can additionally suffer accelerated ageing with further deterioration in the values. This problem is also known in the case of polyurethane or polyisocyanurate insulation panels.
The extent of near-surface foam defects can be very efficiently influenced through the choice of foam stabilizer. Polyether siloxanes having so-called endblocked polyether side groups, i.e., polyethers which instead of an OH group have a terminal alkyl ether or ester group, are known for comparatively defect-free surface qualities. Unfortunately, these foam stabilizers are less soluble in polyol formulations than OH-functional products. The use of insoluble foam stabilizers in preformulated polyol systems of the kind commercially customary for the fields of refrigerator insulation and metal composite elements is ruled out by the risk of phase separation of the formulation during prolonged storage times prior to processing. Therefore, the use of fully endblocked foam stabilizers for improving the surface quality in the case of refrigerator applications, in particular, but also in many other fields of application is only possible to a limited extent, if at all.